1. Field of the Invention
The present invention relates to a process making it possible to control or check the conformity of hybridization balls, particularly of a chip, a chip package, or a substrate. The term conformity of the balls means that the balls have a height between a minimum height (hmin) and a maximum height (hmax). It also relates to a device for implementing said process. It has applications in the fields of microelectronics, data processing and airborne electronics.
2. Description of Prior Art
A procedure for the transfer of electronic components to an interconnection substrate making use of microbosses, protuberances or balls is known to one skilled in the act. This transfer procedure is called flip-chip. According to the flip-chip procedure, the microbosses are made from a meltable material deposited by electrolysis or evaporation, e.g. on the connecting pads of the inputs/outputs of the electronic component. This meltable material can e.g. be indium or a tin-lead alloy. The operation of transferring the electronic component to the substrate takes place at a heating temperature at least corresponding to the melting point of the chosen meltable material. This transfer operation can be likened to a brazing operation.
Such a transfer process is known as C4 (which means Control Collapse Chip Connection). This process has formed the subject of numerous publications and is in particular described in "Micropackaging Handbook" by TUMMALA.
The evermore frequent use of multi-chip modules implies an optimization of the useful substrate surface. It is for this reason that the flip-chip procedure is the most widely used in the case of modular multi-chips. It is more particularly used in wide circulation sectors, where vital significance is attached to costs. The latter constraints have led to the development of processes for producing balls based on electrolysis and which are more economic than the conventional processes using evaporation.
Such electrolysis processes suffer from a major disadvantage namely the difficulty of obtaining a perfect homogeneity of the dimensions of the balls and more particularly their height.
The chips provided with their balls are then assembled with a substrate by deposition on the latter. The following stage consists of a thermocompression (heating+pressure) or a solder reflow, i.e. a heating only of the chip-microball-substrate assembly.
As a result of the pressure exerted, the chip thermocompression operation makes it possible to accept a certain inhomogeneity with respect to the height of the balls prior to assembly. However, this process is individual. This means that it is successively applied to each of the chips deposited on the substrate. Thus, each chip is individually pressed and heated, which for the production of a multichip module (possibly having some 120 chips) requires a very long assembly time. Moreover, it is difficult to ensure a pressure distribution with respect to the balls over chips having a very large number of inputs/outputs without deteriorating the requisite precise positioning.
The solder reflow process is faster to carry out and is therefore less onerous. However, the tolerance of this process is lower with respect to ball height inhomogeneities. It is therefore necessary to check the height of the balls. Different methods have been developed in the industry for checking the presence and the size of the hybridization balls of chips prior to assembly on a substrate.
In most industrial applications, the balls to be checked have a size between 20 and 150 .mu.m and said check must take place with a resolution of approximately 5%. Moreover, for productivity reasons, such a ball size check must take place at a speed close to 150 ms/ball.
A first method consists of high resolution image analysis. This method makes use of a system dedicated to the checking of the masks used in silicon integrated circuit production workshops and operating in planar manner. The resolution obtained by this first method is below 1 micron. However, tests carried out on non-planar structures (which includes balls) demonstrate a speed which is incompatible with the rates used for the checking of hybridization balls in the industry.
A second method uses an autofocussing system able to focus on the one hand on the top of the ball and on the other on the substrate. This method makes it possible to obtain the profile of the balls at a speed of approximately 1 second per ball. However, the obtaining of a complete profile of the balls is not necessary for checking the size of the balls. Moreover, the rate permitted by this method is far from the desired speed of 150 ms/ball.
A third method makes use of microprofilometers such as those used in the semiconductor industry for checking the thicknesses of deposits. These apparatuses make use of the displacement of a light pen on the structures to be measured. An adaptation of these microprofilometers to the checking of the size of balls has been envisaged. However, for very soft material such as indium, the marking of the ball and the dirtying of the light pen are not insignificant. Moreover, the positioning of the light pen must be very accurate and the checking speed which can be attained is not compatible with that required.
A fourth method consists of a laser triangulation. This method, which has the advantage of being very accurate, is complex to perform and too slow to be compatible with the desired speed of 150 ms/ball.
Finally, a fifth method consists of analyzing a projected shadow. According to this method, the analyzing image is the shadow of the ball obtained by a 45.degree. incident illumination. The interest of this method is the "enlargement" of the observed subject. However, as the definition obtained is linked with the topology and the cleanness of the chip surface, said method requires a readjustment of the contrast for each check or control. It can therefore only be of interest for a single surface state. It becomes very complex when there are several surface states.